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24th April 1923, Page 24
24th April 1923
Page 24
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A Consideration of the Problem of Employing Transmitters on Vehicles. A Thirty-mile Range Probably the Maximum Attainable.

ALTHOUGH, AS WAS pointed out in a previous article of this series, it is not likely, so far as one can see at present, that transmitting sets will become the general rule on vehicles in the future, there still may arise exceptional circumstances in which it be desirable that the driver has the means of communieating,with headquarters and, so, in this issue, we will briefly consider the question of transmitters and see how far it is likely that they can be applied to road vehicles. The question of transmitters raises questions under two heads :—There, are the physical possibilities to be considered and what one may term (for want of a better flame)' the difficulties raised by officials. It is well known that small and compact transmitting sets can be .built, as instance the use of such equipment on aeroplanes, but it is quite another question as to what sort of results might. be

obtained when a set of this kind ems ;teed on a commercial vehicle.

The aeroplane set is working under almost ideal conditions for maximum efficiency. The aeroplane is usually two or three thousand feet up in the air, and it is clear of most terrestrial obstructions. The span of the wings of the average commercial aeroplane is always pretty large, so there is no difficulty . in providing a large antennal system by running a wire. all round the leading edge of the planes and trailing another several hundred feet in length under the machine.

Now, it is the size of the aerial which, in transmitting, absolutely determines what proportion of the total energy put into the set in the form or current will be radiated as electro-magnetic waves. We have seen that, even in reception, the larger the aerial system the better the functioning of the set and the louder the signals, other things being equal ; but, evhereas it is perfectly possible to tune a small aerial to receive waves many, hundred of times longer than its natural wave-length and still preserve the efficiency of the arrangement, it is quite another matter when we came to transmissioe. The amount of current represented by even the roudest incoming signal is scarcely measurable in microamperes (that is millionths of an ampere); the value of.the transmitting aerial current often runs into whole amperes. Hence, the ohmic resistance, which, in the case of the reception aerial, is a negligible factor, in the transmitting aerial is of the first importance. This means that to radiate a large amount of energy requires a large aerial. And it is just that which is so difficult to provide within the confines of a commercial vehicle:

The best that could be done is to make use of a six or eight-wire aerial, spread by means of circular or polygonal hoops. Such an aerial is euphemistically known as a " sausage" in wireless parlance. The hoops should be not less than 5 ft. in diameter, and for transmitting it is absolutely essential that the iesulation be of the highest order and also that all stay wires be broken by insulators. This is because all metallic objects near a transmitting aerial act as


ieceiving aerials and absorb a certain amount of energy, unless steps are taken, by breaking them up into comparatively small masses, to minimize this action. It might be asked, why not use the frame aerial; but a frame aerial is out of the question for transmitting purposes, because, although it will reeeive fairly well, it is a very poor radiator of electro-magnetic waves.

In one of the previous articles we pointed out how

thermionie -valve acted, and we stated therein that, by arranging suitable circuits in connection with it, it might be made to switch a current of electricity on and off at an extremely rapid 'Ate. This i5 the way in which we use the valve for transmitting. We set up a cireut containing a valve, which, when supplied with the necessary -current to heat the filament and supply the plate, will have generated in it a continuous series of intermittent pulses of current. The frequency' at which these pulses occur determines -the wave-length of the resultant radiation. The faster the pulses follow one another the shorter the wave-length, and vice versa. In this condition the circuit is said to be oscillating. We cannot hear the effect of these high-frequency oscillations however, even if we were to connect a pair of telephones in the circuit eo that the current pulses flowed through them, because they occur at a frequency far above that detectable by the human ear. However, if we superimpose on these oscillations the slow alternations of current as generated by a microphone (the familiar apparatus into which one speaks on the ordinary telephone) we shall cause the aerial to radiate a distorted wave in place of the pureone that it will send out when left to itself. It is this distorted or modulated wave that affects the aerial of our receiving set and causes it to reproduce the sounds of the voice or the music that we-can hear.

In practice, two valves are generally used, the first one controlling the second, which is oscillating. The filaments of these valves, which are larger than. the receiving pattern, take considerably more current and the plate voltages are very much higher. For the commercial vehicle it would probably be necessary to have a power-driven generator for the plate current, similar to that employed on aeroplanes. The filarffent current would be supplied through accumulators, 'which would need to be considerably larger than those necessary for reception.

It is very doubtful, however, whether, even under the most favourable conditions, the range of the best set that could be put upon a commercial vehicle would be more than thirty miles, so it will be seen that, in the main, a transmitting station on a vehicle is scarcely worth the cost of installation and upkeep.

And on top of all this is the question of whether thepresent or any future P.M.G. -would license vehicles as: transmitting stations!


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